This invention relates to the biological purification of wastewater by mixing the wastewater with activated sludge in an aeration basin in the presence of carrier particles for microorganisms. Such mixtures are aerated with an oxygen-containing gas. Subsequently, the treated wastewater and free activated sludge are discharged from the aeration basin while retaining the carrier particles. Additionally, this invention relates to aeration basins which are used to treat wastewater with activated sludge and carrier particles.
The specific gravity, the size and the macropore properties of these open-cell foam carrier particles are in such a combination that they readily rise and drop in an aeration basin's mixing currents and, therefor, enhance the rate of mass transfer.
A major problem, however, with the use of carrier particles in activated sludge aeration basins has been that the flow of the wastewater-activated sludge mixture from the feed inlet toward the outlet of the aeration basin, results in a gradual conveyance of the carrier particles toward the outlet zone. Thereby, a relatively high concentration of the carrier particles is created near the outlet. This effect results in two apparent disadvantages. First, aeration basin outlets are usually equipped with a screen or perforated plate or similar means for retaining the carrier particles. Consequently, during the course of time, the carrier particles tend to clog or block the outlet. A second deleterious consequence is that the carrier particles are no longer present in adequate numbers in the inlet zone of the aeration basin. At the inlet zone, a high biomass concentration is most desirable.